There is a small group of stars that orbit dangerously close to the supermassive black hole at the center of the galaxy, Sagittarius A*. These S-stars, as they are called, get so close to the maw at the center of the Milky Way that the immense gravity slings them at velocities around 2.5 percent the speed of light, or almost 17 million miles per hour (27 million km/h).

One of these stars, S0-2, will make its closest approach to the black hole at the center of our galaxy this spring. Sagittarius A*—the most massive object in the galaxy with more than 4 million times the mass of the sun—will warp the star and tug at its light, which astronomers can measure from Earth, about 26,000 light-years away from the Galactic Center where the supermassive black hole reigns.

Astronomers have been waiting for the star, known as Source 2 (S2 or S0-2), to make its close pass to Sagittarius A* for 16 years, the length of time it takes the massive star—about 15 times the mass of the sun—to orbit Sagittarius A*. Every orbit, it makes a close pass some 18 billion kilometers from the black hole. At its closest approach, which is in a few short months, S0-2 will creep up to the event horizon of Sagittarius A*, the border from which not even light can escape, at a distance that is only about 2.5 times the distance from the sun to Pluto.

The galactic event offers a perfect opportunity to test Einstein's theory of general relativity. According to the theory, which has formed the backbone of astrophysics for a century, space is not actually empty but more of a "fabric" that is warped by massive objects, which produces gravity. A common visualization is a bowling ball on a trampoline, which would stretch the material, and anything you placed on the trampoline would roll toward the bowling ball—except this process takes place in four dimensions for stars and black holes. Strong enough gravity, therefore, warps space-time, stretching distances and slowing down time.

An image of Sagittarius A* taken with NASA’s Chandra X-Ray Observatory. The circled sections show light echoes from the supermassive black hole.

NASA/CXC/Caltech/M.Muno et al.

When S0-2 makes its close pass to Sagittarius A*, the supermassive black hole will create a "gravity well" that the star's light needs to climb out of. As a result, the light from S0-2 should be shifted into the redder parts of the electromagnetic spectrum. Redshift also occurs when an object is moving away from the observer, but gravitational redshifting is a specific phenomenon predicted by Einstein.

"It will be the first measurement of its kind," said co-author Tuan Do, deputy director of the Galactic Center Group at UCLA, in a press release. "Gravity is the least well-tested of the forces of nature. Einstein's theory has passed all other tests with flying colors so far, so if there are deviations measured, it would certainly raise lots of questions about the nature of gravity!"

Tuan Do and a team of astronomers using the Keck Observatory on the volcano Mauna Kea in Hawaii recently conducted observations to confirm that S0-2 does not have a binary partner star, as many of the S-stars that orbit Sagittarius A* do. The finding comes as a relief for astronomers gearing up to observe the close pass of S0-2, as a pair of binary stars would introduce additional parameters and make the measurements more complicated.

The UCLA Galactic Center Group during a visit to Keck Observatory, located atop Mauna Kea, Hawaii. Members of the group will return to the Observatory this spring to begin observations of S0-2 as the star travels towards its closest distance to the Galactic Center’s supermassive black hole.

UCLA GALACTIC CENTER GROUP

"This is the first study to investigate S0-2 as a spectroscopic binary," said lead author Devin Chu, an astronomy graduate student at UCLA. "It's incredibly rewarding. This study gives us confidence that a S0-2 binary system will not significantly affect our ability to measure gravitational redshift."

Most of the S-stars in the vicinity of Sagittarius A* are binary stars, so the team will continue to survey these stars to learn more about how they form, and why S0-2 formed without a large binary partner.

Sixteen years ago, the last time S0-2 descended close to the void, astronomers knew of the star's existence, but they lacked observatories powerful enough to measure the gravitational redshift. This time around, Einstein's theory of gravity will be put to the test will more precision than ever before.

The general theory of relativity was a revelation in the scientific world, but since its widespread acceptance, some minor discrepancies have crept into the study of physics. The quantum behavior of particles, in particular, seems to flout Einstein's law. With the upcoming observations, astronomers will push on the theory harder than ever before to see if it holds.

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